The Swiss assisted-suicide organisation Dignitas was fiercely criticised by medical and legal experts for "inhumane" practices yesterday after its new method for helping people to kill themselves using a plastic bag and gas was revealed.

According to Swiss prosecutors, the Zurich-based group has assisted in four cases in the past month in which people have used this suffocation technique.

Ludwig Minelli, founder of Dignitas, did not comment on yesterday's reports, but has described the method as a "faster" way of killing oneself. It excludes the need to obtain a doctor's prescription. Previously, those wishing to take their lives with the organisation's help had swallowed pills used in veterinary medicine.

The chief prosecutor, Andreas Brunner, said he had a video of a suicide in which the dying person's body "shook for well over 10 minutes".

A claim that the new method led to a painless death was questionable, said Karin Keller-Sutter, a forensic expert. "No one knows what the dying person goes through once they have lost their consciousness after one to two minutes."

Brunner, a vocal critic of Dignitas, said he was particularly concerned that doctors had effectively been excluded from the process of assisted suicide through the introduction of the new method.

The practice was condemned as "inhumane" by Germany's Hospice Foundation, which calls for all commercial mediation of assisted suicide to be outlawed.

Since it was set up 10 years ago, Dignitas is believed to have helped more than 700 people, the majority of them Germans, to kill themselves. Assisted suicide has been legal in Switzerland since 1942, as long as a doctor has been consulted and the person wishing to die is judged to be fully aware of his or her decision.